Each "Web Application (WAR)" Resource should have one or more child "Web Application Context" Resources (depending on how many different vhosts it is deployed to). It is these ""Web Application Context" Resources that will have all of the numeric metrics - active sessions, avg response time, etc...

Yes, it would be possible to add your own custom Resource types, exposing whatever metrics, operations, and/or configuration you want. You would do that by creating your own RHQ plugin and dropping the plugin jar into the admin-console war's plugins directory. For details on how to write an RHQ plugin, see http://www.rhq-project.org/display/RHQ/Plugin+Community.

The admin console does not save historical metric data or provide any facilities to do so. For that, you'll want to install RHQ (or, for production purposes, JBoss Operations Network (JON) which is the product based on RHQ).